Plasma and Trap-Based Techniques for Science with Antimatter J
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Plasma and trap-based techniques for science with antimatter J. Fajans1,a and C. M. Surko2,b 1 Physics Department, University of California Berkeley 2 Physics Department, University of California San Diego ABSTRACT Positrons (i.e., antielectrons) find use in a wide variety of applications, and antiprotons are required for the formation and study of antihydrogen. Available sources of these antiparticles are relatively weak. To optimize their use, most applications require that the antiparticles be accumulated into carefully prepared plasmas. We present an overview of the techniques that have been developed to efficiently accumulate low energy antiparticles and create specially tailored antiparticle plasmas. Techniques are also described to create tailored antiparticle beams. Many of these techniques are based on methods first developed by the nonneutral plasma community using electron plasmas for increased data rate. They have enabled the creation and trapping of antihydrogen, they have been critical to studies of positron and positronium interactions with matter, including advanced techniques to characterize materials and material surfaces; and they have led to the creation and study of the positronium molecule. Rather than attempting to be comprehensive, we focus on techniques that have proven most useful, applications where there has been significant, recent progress, and areas that hold promise for future advances. Examples of the latter include ever more precise comparisons of the properties of antihydrogen and hydrogen, tests of gravity using antihydrogen and positronium atoms, and efforts to create and study phases of the many-electron, many-positron system. currents, only pico-Coulombs at sub-pico-amp currents are I. INTRODUCTION available in the case of positrons. Antiprotons for low-energy research with antimatter are available only at the Antiproton In the past few decades, the use of antimatter for scientific and Decelerator (AD) [16] at CERN in Geneva, Switzerland. Once technological purposes has become increasingly important. degraded to below 5 kV, bunches of only ~ 105 antiprotons are Positrons are used to characterize materials and material surfaces delivered by the AD, at a rate of one bunch every two minutes. [1], and for positron emission tomography (PET) which is used in The new upgrade to the AD, ELENA [17], is expected to deliver drug design and to study metabolic processes [2]. Scientific 10–100 times more useable antiprotons. applications include tests of quantum electrodynamics (QED), creation of exotic species such as positronium (Ps) and the Some applications demand tailored antiparticle beams. + - + - positronium molecule (e e e e , symbol Ps2) [3, 4], and Depending on the application, one might want fine lateral understanding fundamental positron interactions with ordinary focusing, high areal densities, low-energy beams, nearly matter including atoms and molecules [5, 6]. One of the newest monoenergetic beams, or short temporal pulses. Alternatively, developments is the ability to create high-quality beams of one might want to deliver intense bursts of large numbers of positronium atoms for precision measurements and for antiparticles. fundamental physics tests, such as the gravitational attraction of antimatter to our (matter) earth [7, 8]. Other applications work best with confined antiparticles. Because antiparticles suffer annihilation when they come in Antiprotons play a central role in the formation and study of contact with matter, they must be confined in vacuum, typically antihydrogen (the bound state of the antiproton and the positron in an electromagnetic trap. The antiparticles form a charged and the simplest stable antiatom). Antihydrogen is being used to cloud which is often in the plasma state. The focus of this article test the CPT theorem (i.e., the predicted invariance of relativistic is to describe the techniques required to accumulate antiparticles quantum field theories under charge conjugation, parity inversion and manipulate the resulting plasmas, tailored for specific and time reversal) and the gravitational attraction of antimatter to applications. The techniques described here rely heavily on matter. Results have been obtained for the 1S-2S transition [9], research in plasma and beam physics [15]. In particular, many and the hyperfine transition [10], which, by an absolute energy useful processes are extensions of techniques developed to tailor metric [11], are some of the most precise tests to-date of the CPT more conventional single-component plasmas (i.e., plasmas theorem. Crude measurements of the interaction of antihydrogen composed of electrons or ions) and mixed-species nonneutral with the earth’s gravitational field have also been performed [12]. plasmas. CPT tests such as a comparison of the proton/antiproton magnetic moment and mass have also been performed with isolated II. ANTIMATTER PLASMAS IN TRAPS antiprotons [13, 14]. These tests have attracted much attention, both in the physics community and with the lay public. A. Penning-Malmberg traps Sources of antiparticles are relatively weak. Positrons can be A wide variety of electromagnetic traps have been used to confine obtained from a variety of radioisotopes, nuclear reactors, and positrons, including Penning traps, magnetic mirrors and levitated linear electron accelerators (LINACS) [15]. However, while one magnetic dipoles [18-22]. For long-time confinement of large can easily obtain many Coulombs of electrons at amp-strength numbers of positrons or antiprotons, the method of choice is some a [email protected] b [email protected] 1 variant of the Penning-Malmberg (PM) trap [23]. As shown in asymmetries [28]. Recent evidence suggests that colloidal- Fig. 1, PM traps use a uniform magnetic field for radial graphite-coated electrodes are superior to electroplated gold in confinement and an electrostatic potential well in the magnetic minimizing patch asymmetries [29]. field direction for axial confinement. These traps are used to confine gases or plasmas whose constituents are all of the same In practice, plasma confinement times in PM traps range from charge sign, though in antihydrogen synthesis, two adjacent, milliseconds to hours and scale approximately as [27]. There 1 oppositely charged plasmas are merged. As pointed out by is evidence that confinement is superior in multi-ring2 PM traps O’Neil, for a cold, magnetized plasma consisting of particles with [30], which utilize many short electrodes extending over the a single sign of charge, the canonical angular momentum in a PM length of the plasma, rather than one long electrode, as depicted trap can be approximated as in Fig. 1. These short electrodes can be used to generate a near- harmonic potential. Investigation of the possibly better , (1) performance of such multi-ring traps is a fruitful area for further research. 2 2 where z is the direction ≈ of∑ the magnetic field B, and is the radial B. Ultra-long-time confinement position of particle j with charge e (SI units) [24]. If there are no torques on the plasma, the angular momentum is constant and the If long-time confinement is needed, antiparticles can be plasma cannot expand. Thus, confinement is nominally perfect, transferred to an ultra-high vacuum (UHV) PM trap where and the plasma can reach an equilibrium state [25]. annihilation losses are minimized (cf. Fig. 2) [31]. Transfer efficiencies can be in excess of 90 %, but can also be lower depending upon the specific circumstances. Antimatter can be routinely confined in such traps for days, and in exceptional cases, years [13, 32], using traps mostly or entirely enclosed by surfaces at 4.2 K. Pressures below 10-14 Torr are readily obtained in such cryogenic traps, and can go as low as ~10 Torr [13, 32]. When necessary, plasma expansion can −be18 minimized or eliminated by applying rotating electric fields [i.e., the “rotating Fig. 1. Schematic diagram of a Penning-Malmberg trap for the wall” (RW) technique [33]]. The RW technique and long confinement of plasmas consisting of particles of a single sign of confinement also require good particle cooling, which can be charge, here biased for positive charges. Typical electrode radii provided by cyclotron radiation in strong (e.g., tesla-strength) and lengths are several centimeters. The “parallel” direction z is magnetic fields. We defer further discussion of the performance defined to be aligned with the trap and magnetic axes, and and limits of UHV traps to the later sections on cyclotron cooling “perpendicular” refers to the orthogonal directions. and the RW technique. A plasma in a PM trap produces a strong radial electric field. This C. Buffer-gas PM traps field results in an E x B drift in the azimuthal direction, which causes the plasma to spin about the magnetic axis. With good Sources of positrons typically produce particles with energies of confinement, the shears in the plasma damp out, and the plasma kilo-electron volts or higher. There is not yet an efficient way to rotates as a rigid rotor at frequency trap particles at these energies, and so various materials (“moderators”) are used to slow them to electron volt energies [1, = , (2) 15, 34-36], whereupon they can be trapped in a buffer-gas trap (BGT). The BGT (cf. Fig. 2) is a modified PM trap that employs 40 where is the plasma density [26] and is the permittivity of a stepped potential well in the B direction and corresponding free space. Depending upon the application, PM traps can operate regions (stages) of varying gas pressure. The highest-pressure at a variety of magnetic fields (e.g., 0.01–70 tesla). As discussed in region (stage I) is used to trap the particles by electronic excitation the next section, particle cooling is frequently necessary. At high of a molecule (N2 is the molecule of choice) in one transit through (e.g., tesla-strength) magnetic fields, naturally occurring the trap. Subsequent collisions act to move the particles to stages cyclotron radiation can fill this role, while at low B, other of lower potential and gas pressure, where annihilation is slower techniques, such as collisions with a molecular gas, are used.